1. Field of the Invention
The invention relates generally to electrical discharge machining (EDM), and more specifically to a method and an apparatus for generating machining pulses for electrical discharge machining, such as wire-cutting and die-sinking EDM, in the range of medium-machining accuracy or roughness between rough-machining and fine-machining.
2. Related Technology
The range of medium-machining accuracy, i.e. with a surface roughness Ra in the range of 0.15 μm to 0.8 μm has, in the past, been neglected by manufacturers of EDM machines in favor of maximizing erosion performance and finest surfaces. This is not justified, for one thing already, because the times of all steps in machining add up, and, for another, because poor machining quality effects subsequent machining steps over-proportionally. Since the rough-machining of the workpiece generates only a very inaccurate surface, whereas the fine-machining can be implemented only with the small erosion rates, it would be totally uneconomical to directly change from rough-machining to fine-machining.
In the pulse generator known from JP 07-266 133A (OIZUMI), as shown in FIG. 2, coaxial cables A, B are connected via terminal resistors ZA, ZB having matching impedances between an electrode E and a workpiece W. The coaxial cables A, B are charged by a voltage source U via a transistor T, a load resistor R and the terminal resistors ZA, ZE. A frequency generator G generates at the transistor T the desired pulse-on/off times for the charging process. When sparking between the electrode E and the workpiece W occurs, the coaxial cables A, B are discharged over the terminal resistors ZA, ZB in a theoretically square pulse whose duration is determined by the length of the coaxial cables and whose amplitude is a function of the voltage U of the voltage source and the line impedance of the coaxial cables. This known configuration has, however, the following drawbacks, for instance: The charge circuit has firstly the disadvantage that it only works when the terminal resistors ZA, ZB precisely correspond to the line impedance of the coaxial cables A, B. Since the discharge circuit formed by the electrode E and workpiece W also comprise a stray inductance as well as a stray capacitance, however, unavoidable reflections occur, resulting in sinusoidal, bipolar pulses and in corresponding high electrode wear. A second drawback exists in the way the no-load voltage is generated. When the spark or working gap is too large, then no more discharges should occur so as not to detriment the workpiece geometry by sparkout. However, it is just this sparkout that occurs with this arrangement, because the charged coaxial cables A, B, despite the interrupted transistor T, apply a DC voltage to the working gap. A further problem is involved in the arrangement of the terminal resistors ZA, ZB themselves, via which charging and discharging the coaxial cables occurs, which firstly have a very modest efficiency of between 50% and 2.3% depending on the pulse amplitude and cable impedance, and secondly resulting in merely half the pulse amplitude at the spark gap. A further drawback is the high power loss of the terminal resistors ZA, ZB of e.g. 100 kW pulse power at a current amplitude of 100 A and a cable impedance of 10Ω occurring directly in the working zone, possibly resulting in thermally induced deformation of the machine structure and thus in additional errors. Also unpractical is accommodating the quite considerable length of the coaxial cables for longer pulses on the machine. This is why the generator known from JP 07-266 133A is unsuitable for medium accuracy machining.
A similar discharge pulse generator for generating machining pulses for electrical discharge machining is disclosed in U.S. Pat. No. 6,566,823 (Kinbara). Also here the generator circuit comprises a resistor having a resistance value equal to the characteristic impedance of the energy discharging coaxial lines in order to provide an impedance matching. Thus, the drawbacks mentioned above apply for this generator as well.
So-called line or cable generators using the principle of generating ultrashort high-energy pulses by the discharge of delay lines are known since more than 60 years. A detailed explanation of the principle of line generators, by way of examples for high-energy generators, is disclosed in the textbook “Impulse in der Grenzphysik”, W. Bartel, et al., Oldenbourg Verlag, Munich 1976, pages 132 to 134. In the same book, pages 135 to 137, the principle of the Blumlein line generator (see also GB 589 127, filed on Oct. 10, 1941) is also described. This generator comprises two lines which when loaded are definitely mismatched to twice the line impedance. Because of the mismatch and the resulting reflections two part-pulses add up into a total pulse having a duration corresponding to twice the delay time but with the full amplitude. The Blumlein generator would thus eliminate the main drawback of JP 07-266 133A which can only attain half the amplitude. Unfortunately the stray inductance and stray capacitance of the discharge circuit disturb functioning of the Blumlein generator to such a considerable extent that it is of no interest for the present application.
For the sake of completeness, mention is also made of EP 0 313 049 B1 (Marsicovetere et al) disclosing a technique to decouple the coaxial cables by switches or diodes from the spark gap in EDM fine-machining. This document makes, however, no contribution to medium accuracy machining.
Thus, there is an need to provide a method and an apparatus for generating machining pulses for electrical discharge machining (EDM) which are better suitable for machining in the medium-machining range, i.e. between rough-machining and fine-machining.